The Scordisci, a Celtic tribe that invades Greece during the first part of the third century BCE, finally settles east of Sirmium at the junction of the Savus and the Danube rivers. (Some Roman authorities consider them a Thracian stock, because of their admixture with an older Thraco-Illyrian population.)

Burebista, in battles mentioned by Strabo, defeats the Celts who menace his western borders after 48 BCE, forcing them back westward into Pannonia, a region originally peopled by the Pannonii (sometimes called Paeonii by the Greeks) and invaded from the fourth century BCE by various Celtic tribes.

Marcus Vinicius, born the son of a Roman knight at Cales in Regio I (Latium et Campania) of Italia, had distinguished himself as legatus Augusti pro praetore (governor) of the Roman province of Gallia Belgica in 25 BCE, when he led a successful campaign into Germania.

At some point, Vinicius may also have served as governor of the Roman province of Achaea; an inscription from Corinth, dated to 18-12 BCE and honoring his fellow-general, and the Emperor's right-hand man, Agrippa, reveals that an administrative division of the city had been named the tribus Vinicia, apparently in Vinicius' honor.

In recognition of his services, Vinicius, the archetypal homo novus, had been appointed suffect consul in 19 BCE, replacing C. Sentius Saturninus and holding the office together with Q. Lucretius Vespillo.

After his consulship, Vinicius continues to be entrusted with important military commands.

Starting in 14 or 13 BCE, Vinicius serves as governor of Illyricum where he is in charge of the early stages of the Roman conquest of Pannonia (the bellum Pannonicum, 14 - 9 BCE) until Augustus' stepson and future successor as Emperor, Tiberius, assumes overall command.

With the Scordisci as allies, Vinicius takes Sirmium.

During or shortly after this war, he becomes the first Roman general to campaign on the far side of the river Danube: he routs an army of Dacians and Bastarnae and subjugates the Celtic tribes of the Hungarian Plain.

Throughout his life, Vinicius seems to have enjoyed a close friendship with the emperor: the historian Suetonius quotes a letter by Augustus in which he talks about playing dice with Vinicius and his fellow homo novus, Publius Silius Nerva.

Tiberius consolidates Roman control of the western Balkan peninsula; by the time of his accession in 14, Rome has subjugated the Celts in what is now Serbia, their conquest of Pannonia complete with the capture in this year of Sirmium (Sremska Mitrovica, Vojvodina, Yugoslavia), the key town of the Sava Valley.

After the revolt is put down, Pannonia is organized as a separate province and garrisoned with three legions.

The Pannonians are mainly Illyrians, but there are some Celts in the western part of the province.

The Iazyges defeat and kill Marcus Claudius Fronto, Roman governor of Lower Moesia, in 170.

Operating from Sirmium (today Sremska Mitrovica, Vojvodina, Serbia) on the Sava river, Marcus Aurelius moves against the Iazyges personally.

After hard fighting, the Iazyges are pressed to their limits.

However, while the Roman army is entangled in this campaign, making little headway, several tribes use the opportunity to cross the frontier and raid Roman territory.

Maximinus, having secured the German frontier, at least for a while, sets up a winter encampment at Sirmium in Pannonia, and from that supply base fights the Dacians and the Sarmatians during the winter of 235–236.

Maximinus spends most of his reign fighting invading tribes along the Danube and the Rhine.

The numerous milestones displaying his name attest to his energetic reconstructions of the roads in these regions.

The Pannonias, with Valerian’s capture in the East, fall under grave threat from the Germanic tribes, and Regalianus, one of the usurpers proclaimed by the Pannonian legions, dies fighting the invaders.

The defense is concentrated around Sirmium and ...

Gallienus, now reigning alone, faces multiple invasions and several usurpations.

He moves constantly between the Danube and the Rhine, there fighting a series of campaigns against the Goths.

A latter plague incident from 270 involves the death of Emperor Claudius II Gothicus, though it is unknown if this was the same plague as the so-called Plague of Cyprian or a different outbreak.

The Historia Augusta (unreliable, but the only history we have for this age) states: "in the consulship of Antiochianus and Orfitus the favor of heaven furthered Claudius' success. For a great multitude, the survivors of the barbarian tribes, who had gathered in Haemimontum, were so stricken with famine and pestilence that Claudius now scorned to conquer them further.... during this same period the Scythians attempted to plunder in Crete and Cyprus as well, but everywhere their armies were likewise stricken with pestilence and so were defeated."